As field effect transistors grow ever smaller, problems begin to arise because of the very short channels that have to be used. In particular, the punchthrough resistance of these devices is often very low. A solution to this problem that has been widely adopted in the industry has been to include an additional implant close to the low density drain. In order to achieve uniformity across the full wafer, it is usual to rotate the wafer during the implantation. As a result, the prior art practice has generally been to simultaneously form a pair of such `pocket` implants, one near the source and one near the drain as this happens automatically. From a performance standpoint only the implant close to the drain is needed so many processes end up with two pocket implants even though the space taken up by the implant close to the source is valuable real estate, particularly as devices continue to grow smaller. It is therefore desirable to form an asymmetric device having a pocket implant only on the drain side. While this is known in the prior art, forming such an asymmetric arrangement has required additional process steps. The method of the present invention allows asymmetric design to be implemented without these additional steps.
In searching the prior art we have been unable to find a process that achieves a single sided pocket implant in the manner taught by the present invention. The following references have, however, been found to be of interest:
Burr et al. (U.S. Pat. No. 5,650,340 July 1997) describes a device having an asymmetric pocket implant under either the source or the drain. Shrivastava (U.S. Pat. No. 5,518,942 May 1996) shows a large angle single sided implant under the source. Kao et al. (U.S. Pat. No. 5,492,847 February 1996) as well as Hsu ( U.S. Pat. No. 5,554,544 September 1996), Pan (U.S. Pat. No. 5,595, 919 January 1997), Huang (U.S. Pat. No. 5,618,740 April 1997), and Tsai et al. (U.S. Pat. No. 5,668,024 September 1997) all show pocket implants using various implant processes including large angle ion implantation.
Han (U.S. Pat. No. 5,409,848 April 1995) and Fratin et al. (U.S. Pat. No. 5,712,814 January 1998) also teach examples of large angle ion implants.